This invention relates to chemical filters, and, more particularly, to a chemical filter for protecting electronic equipment, such as hard discs for computers, from pollutants in the air.
Atmospheric corrosion of ferromagnetic materials by pollutant molecules such as SO.sub.2, H.sub.2 S, NO.sub.2 and Cl.sub.2 is a serious problem in many plant environments in which computer equipment is used, such as for process control. Examples of such plant environments include paper mills and oil refineries. While the level of pollutants at such plants may be within OSHA standards, nevertheless, even pollution levels in the range of parts per billion, far below what is permissible for humans, is detrimental to components in computer equipment, such as metals employed in electrical connections on printed circuit boards and in disc drives. Importantly, ferromagnetic materials, such as cobalt, used in disc drives are adversely affected by pollutants.
In hard disc drives employing cobalt, for example, the chemical reaction of the pollutants with the metal serves to form corrosion products such as oxides and chlorides, which have a larger crystallographic cell structure than the metal itself. This occurs as a result of water vapor from the atmosphere, which forms a thin film, about 100 to 200 .ANG.ngstroms thick. The pollutants dissolve in the water film, forming an acid electrolyte, which corrodes the metal. As a consequence of the formation of the corrosion products, the disc drive head, which is positioned to travel at a high rate of speed very close to the metal surface, hits the expanded corrosion products, called "crashing", with consequent damage to the head and possible loss of data.
A further problem includes organic contaminants in the atmosphere, which are readily polymerized by cobalt and cobalt oxide, well-known for their catalytic properties. The presence of organic polymers causes a problem termed "stiction", in which the head of the disc drive adheres to the organic film and is subsequently torn off during operation of the disc drive.
Attempts have been made to minimize the corrosion problem by forming a protective film over the metal surface. However, the protective film has been found to reduce the signal from the magnetic domains in the metal film.
The solution described above has been applied to disc drives which are primarily sealed but nonetheless communicate with the outside atmosphere. In such units, the flow of air to the hard disc unit is on the order of 15 to 100 ft.sup.3 over the lifetime of the unit. The flow of air is considered necessary for maintaining the internal pressure the same as the ambient barometric pressure. In many environments, this is sufficient to cause corrosion failure.